In-flight refueling (or air-to-air refueling) is an important method for extending the range of aircraft traveling long distances over areas having no feasible landing or refueling points. Although in-flight refueling is a relatively common operation, especially for military aircraft, the aircraft to be refueled (e.g., the receiver aircraft) must be precisely positioned relative to the tanker aircraft in order to provide safe engagement while the fuel is dispensed to the receiver aircraft. The requirement for precise relative spatial positioning of the two rapidly moving aircraft makes in-flight refueling a challenging operation.
There are currently two primary systems for in-flight refueling. One is a hose and drogue system, which includes a refueling hose having a drogue disposed at one end. The hose and drogue are trailed behind the tanker aircraft once the tanker aircraft is on station. The pilot of the receiver aircraft then flies the receiver aircraft to intercept and couple with the drogue for refueling. Another existing system is a boom refueling system. The boom refueling system typically includes a rigid boom extending from the tanker aircraft with a probe and nozzle at the distal end. The boom also includes airfoils controlled by a boom operator stationed on the refueling aircraft. The airfoils allow the boom operator to actively maneuver the boom with respect to the receiver aircraft, which flies in a fixed refueling position below and aft of the tanker aircraft.
Another challenge associated with tanker aircraft stems from the fact that the tanker aircraft deploy the hose and/or boom during refueling operations. If the hydraulic system that powers actuators used to deploy and retract the hose and/or boom fail, the aircraft must rely on a redundant system to stow these devices before landing. Accordingly, tanker aircraft typically include redundant systems that provide this capability. However, a drawback with such redundant systems is that they add weight and complexity to the tanker aircraft and can accordingly reduce the range of the aircraft and/or the overall operational efficiency of the aircraft.
FIG. 4 is a schematic illustration of a portion of an aircraft hydraulic system 10 that is used on existing aircraft, including tanker aircraft. The hydraulic system 10 can provide hydraulic power to an aircraft parking brake 12. Accordingly, the hydraulic system 10 can provide power to the brake 12 via a simple accumulator 30 after hydraulic pressure has been depleted. A control valve 18 controls the application of hydraulic fluid to the brake 12. The pressurized hydraulic fluid is provided by an aircraft hydraulic supply line 13 (via a check valve 15) and returned via an aircraft hydraulic return line 14. The accumulator operates against pressure provided by a gas charge 17 so as to store energy for use when the aircraft is powered down. In other installations, a similar (simple) accumulator can be used to store hydraulic power for high demand periods. While the foregoing hydraulic system is suitable for its intended use, it does not address the weight and complexity associated with redundant systems described above.